Kano model
The Kano model (Noriaki Kano, 1984) categorises product features by their non-linear relationship to customer satisfaction. Five categories: Basic (expected; absence dissatisfies, presence doesn't delight), Performance (linear, more is better), Excitement (unexpected delighters), Indifferent (no effect), and Reverse (unwanted).
The model's value is that it explains why feature lists aren't all created equal. A team that ships a beautiful onboarding feature (Excitement) but fails on password reset (Basic) will get worse satisfaction scores than a team that ships only the basics, because Basic features are dissatisfiers when missing, not satisfiers when present. The discovery technique uses a Kano survey: pair each feature with a functional question ('how would you feel if the product did X?') and a dysfunctional question ('if it didn't do X?') to classify features. Limitation: feature categories drift over time, yesterday's Excitement becomes today's Performance becomes tomorrow's Basic. Kano analysis is a snapshot, not a permanent classification.